Artificial muscle actuators are used in various applications, including robotics, biomimetic systems, medical devices, and aerospace systems. Artificial muscles are designed to approximate biological muscles in shape and motion. They function as tensile actuators, usually with little or no ability to apply compressive force, and, as a result, may be constructed from soft and/or lightweight materials. Among the fluidic artificial muscles, which are controlled by the application of pressurized gas or liquid, force-to-weight and power-to-weight ratios can be quite high, but maximum contraction is usually limited to 40% and often less.
The most common type of fluidic muscle, the “McKibben muscle”, is relatively easy to manufacture. The McKibben muscle consists of a tubular bladder, made of a material like rubber, with a pressure inlet at one end, enclosed in a braided, bias-woven fiber sleeve. Each fiber of the braided sleeve winds in a helical path around the bladder. The bladder and sleeve are clamped together at each end. When the bladder is pressurized, it presses outward on the sleeve, causing the sleeve to expand radially, increasing the fiber angle and decreasing the length of the muscle as the internal volume increases. Variations of this design include combining the bladder and sleeve by rubberizing the sleeve, and leaving the bladder ends disconnected from the sleeve. Various end fittings have been devised to transmit tensile force from the sleeve fibers into the mounting points. Various versions of McKibben muscles are available commercially from companies including Festo and the Shadow Robot Company.
An ideal McKibben muscle is considered to contract theoretically up to 42%, at which point the volume contained inside the shell reaches a maximum. Further contraction reduces the volume of pressurized fluid it holds. However, in practice, McKibben muscles are often found to contract by about 30%. The motion is generally similar to that of a biological muscle on the ascending limb portion of its motion range. This limitation requires significant extra space to be allocated for the actuator, which can be cumbersome and limit the usefulness of the device.
Pleated pneumatic artificial muscles use a bladder material with high stiffness and tensile strength, with no braided sleeve, and numerous axial folds that confer a pleated tube shape when stretched, and a spherical shape when inflated. The maximum theoretical contraction has found to be 54%, and the practical maximum is 45% for this type of muscle.
The Paynter knitted muscle works on the same basic principle as the pleated pneumatic artificial muscle, but is designed with a separate bladder and sleeve, each having tubular ends and a roughly spherical expanded section at the center. The bladder is an elastomer and the sleeve is made by knitting a stiff fiber like Dacron or Kevlar in a tubular pattern, with looser stitches in the middle to make the expanded section. The bladder is disposed within the sleeve and pressurized to fully expand until stopped by the sleeve, then the bladder and sleeve are bonded together with an adhesive. The adhesive bonding prevents sliding of the fiber strand crossings in the sleeve, preserving the wide, square-shaped stitches near the center and tight stitches near the ends. Pleats form irregularly when the actuator is stretched, but can be improved by heating within a pleated mold.
Given the limitations of the various conventional systems, there is a demand for an artificial muscle having an improved ability to contract. The present invention satisfies this demand.